Tumor hypoxia promotes resistance to radiotherapy and chemotherapy, and selects for the emergence of highly malignant cells. The Hypoxia Inducible Factors (HIFs) are the primary transcriptional regulators mediating cellular adaptation to tissue hypoxia, and HIFs are highly expressed in many human cancers. HIFs contribute to tumor progression and metastasis by promoting the growth, metabolism, and survival of cancer cells, as well as modulating the recruitment and function of tumor-associated stromal cells. HIFs have therefore been proposed as an attractive therapeutic target, and significant effort has been focused on identifying HIF inhibitors for cancer therapy. The rationale for this approach has become controversial, however, as distinct HIF complexes have been recently shown to either promote or suppress tumor growth and progression in different cellular contexts. The majority of HIF responses are mediated by two independent transcriptional complexes containing either HIF-1? or HIF-2?, each of which binds the constitutively expressed subunit ARNT (also called HIF-1). Although they fulfill some redundant functions, HIF-1? and HIF-2? also antagonize each other's effects on tumor metabolism, angiogenesis, and immune responses. To date, genetically engineered murine cancer models have investigated the effects of deleting only HIF-1? or HIF-2? independently, without assessing the functional importance of the remaining HIF-? subunit, which may act to either promote or suppress tumor progression. Consequently, the impact of full HIF-deficiency in cancer cells and associated stromal cells cannot be accurately predicted, and could result in unintended tumor progression in specific settings. In this proposal, we will use tissue-specific Cre expression to delete the Arnt/Hif-1 gene (and both HIF-1? and HIF-2? function) in murine colon carcinoma cells and tumor-associated macrophages (TAMs). This approach will, for the first time, model the impact of pan-HIF inhibition in an autochthonous murine tumor model. This work will also inform ongoing efforts to develop pharmacological HIF inhibitors as safe and effective cancer therapies.